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Swift finds the recurrent nova T Pyx has become X-ray bright and variable

ATel #3647; E. Kuulkers (ESA/ESAC), M. Orio (INAF & U. Wisconsin), J. P. Osborne, K. L. Page, A. P. Beardmore (U. Leicester), P. Kuin (MSSL), S. Balman (METU), M. F. Bode, Darnley, M. J. (LJMU), J. J. Drake (CfA), P. A. Evans (U. Leicester), S. P.S. Eyres (U. Central Lancashire), J. Krautter (LSW Heidelberg), J.-U. Ness (ESA/ESAC), T. J. O'Brien (U. Manchester), B. E. Schaefer (Louisiana State U.), G. J. Schwarz (AAS), S. Shore (U. Pisa), S. Starrfield (Arizona State U.), D. Takei (CfA), J. W. Truran (U. Chicago), F. M. Walter (Stony Brook U.)
on 15 Sep 2011; 07:58 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

Subjects: Ultra-Violet, X-ray, Binary, Cataclysmic Variable, Nova, Transient, Variables

Referred to by ATel #: 3679, 3707, 3782, 4452

The recurrent nova T Pyxidis, which is currently in outburst (IAUC # 9205 ), continues to be monitored by Swift at X-ray and UV wavelengths after its emergence out of the Swift Sun-constraint (see ATel #3549). The Swift/XRT observations (ATel #3285, ATel #3549) between UT 2011 Sep 3-11 (142-150 days after discovery on 2011 Apr 14) shows a source at 0.3-1 cts/s (0.3-10 keV). This is an order of magnitude brighter than seen just before the source entered the Swift Sun-constraint (2011 Aug 16, or day 124: ATel #3549). The source varies within the above quoted count rates on a time scale of hours.

The increase in brightness is not only evident at soft energies as would be expected from the unveiling hot white dwarf. The flux has increased at both low (0.3-0.5) and higher (1-10 keV) energies, although the source has not been detected above about 5 keV. This hard X-ray rise in correlation with the soft X-ray does seem quite unusual compared to that previously seen in (recurrent) novae. The higher-energy increase is a factor of about 4 less than that seen in soft X-rays. We note that a change in the hydrogen column density, N_H, is apparently not the cause of this difference. Possibly internal or external shocks continue to develop, contributing to the hard X-ray emission.

When we fit an average Swift/XRT spectrum (from Sep 3-7) by a MEKAL and black-body (BB) contribution, both subjected to absorption, we derive the following parameters: MEKAL kT = 2.3 +/- 0.3 keV, BB kT = 46 +/- 3 eV and N_H = (1.1 +/- 0.2)e21 cm-2. The observed BB flux contribution is about 81% of the total 0.3-10 keV observed flux during this interval. The fit is, however, not perfect (cstat value of 322 for 222 degrees of freedom); especially the hard emission is not well described by this model, where multiple temperatures or non-solar abundances are suggested.

The UVOT shows the current uvm2 (centered at 2246 Angstrom, FWHM = 498 Angstrom) brightness to be about 11 magnitudes on UT Sep 12 12:25 (day 151).

T Pyx will be monitored by Swift for about 1 ksec every day for the next 7 weeks. We thank the Swift operations team and PI for their support.